1. Technical Field
The present invention is directed to a method for increasing the high load (knock) limit of an internal combustion engine operated in a low temperature combustion mode such as a homogeneous charge compression ignition mode.
2. Description of the Related Art
Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke. Traditional gasoline engine combustion results in a premixed turbulent flame, while traditional diesel engine combustion results in a mixing controlled diffusion flame. Both processes are controlled by fluid mechanics, as well as heat and mass transfer. Each type of engine has advantages and disadvantages. In general, gasoline engines coupled with 3-way emissions catalysts produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
These air pollutants are composed of particulates and gaseous compounds including, among other things, oxides of nitrogen (NOx). Continued global emphasis and government legislation on reducing emissions and improving fuel economy of internal combustion engines has led to the need to develop advanced high efficiency, clean combustion engines. Exhaust after-treatment systems (such as Selective Catalyst Reduction (SCR), lean NOx traps, and diesel particulate filters) have been designed and commercialized to lower exhaust emissions of diesel engines to meet emission targets and regulations. However, these systems are costly, add to the weight of the vehicle, increase exhaust back pressure and minimize fuel economy due to the added weight and the need to use fuel to regenerate the systems. Reducing engine-out emissions would decrease the size and/or eliminate the need for these systems.
One approach explored in the industry to simultaneously reduce emissions (compared to a traditional diesel engine) and improve efficiency (compared to a traditional gasoline engine) is to operate the engine at a lower combustion temperature (typically called “low temperature combustion” (LTC)). This can be achieved by premixing some or all of the fuel with air (and optionally recycled exhaust gas), either prior to entering the cylinder, or alternatively in the cylinder well before combustion occurs. This, in turn, greatly reduces (or eliminates) the fraction of fuel that is burned with a mixing controlled diffusion flame (diffusion flames lead to high combustion temperatures). Also, the ratio of the fuel to the total mass in the cylinder is kept low to ensure low temperature combustion. This greatly reduces the effectiveness of a spark plug. As a result, ignition is normally initiated via compression; however, a spark plug can be used to assist. The low temperature after combustion significantly reduces NOx formation due to the fundamental chemistry of the reaction pathway. The use of fuels with gasoline-type volatilities (vs. heavier fuels such as diesel), combined with premixing the fuel and air limits soot production. There are several types of LTC each of which has a distinct acronym, for example: HCCI, PCCI, CAI, PPC, RCCI, MK, UNIBUS, OKP, and the like.
One drawback to these LTC-type technologies is that the speed-load (speed-power output) operating range is very limited, and significantly smaller than required and provided by current gasoline spark-ignited and diesel compression ignition internal combustion engines. Moreover, the high load limits which are typically limited by engine-knocking are especially lower than in conventional engines.
Homogeneous charge compression ignition (“HCCI”) is an attractive advanced combustion process that offers potential as a high-efficiency alternative to conventional spark ignition and compression ignition engines. The operating principle of HCCI engines combines characteristics of spark ignition and diesel engines. The primary mechanism that limits the load (power) output from LTC engines, especially HCCI engines, is combustion noise (i.e., knocking/ringing) which can potentially damage the engine and is due to excessive pressure rise rates (PRR). A large PRR is a direct result of the charge homogeneity: ignition happens everywhere in the cylinder almost simultaneously, unlike in a spark ignition engine where it is limited by flame propagation rates or in a diesel engine where it is limited by mixing and injection rates. Accordingly, the nature of this problem is akin to spark-ignition knock and presents a high-load knock limit to the operating range.
Ongoing R&D efforts have shown that fuel compositions and their properties can have an impact on the speed-load range that can be obtained. With diesel fuel, elevated temperatures are required before significant vaporization occurs making it difficult to form a premixed near-homogeneous charge. Diesel fuel also has significant cool-combustion chemistry leading to rapid auto-ignition once compression temperatures exceed about 800° K. This can lead to overly advanced combustion phasing and/or require reduced compression ratios that reduce engine efficiency.
At the other extreme are high octane fuels like “pump” gasoline (i.e., gasolines available in the marketplace) and isooctane which are easier to volatilize and mix with air, but which are very resistant to autoignition. Typically, they require high inlet temperatures or the addition/recycle of hot exhaust gases to initiate autoignition. As a result, charge densities are much lower than those of typical engines, and the fresh fuel/air portion of the charge is limited.
One possible solution to reduce the foregoing problems is the use of fuels having gasoline-like volatilities, but somewhat lower octane numbers than “pump” gasoline, but are more reactive. Moreover, the combustion phasing for a more reactive fuel could be retarded farther with good stability, which allows higher charge-mass fuel/air equivalence ratio (era) without knock. The combination of a higher φm and higher charge density from low-octane fuels has been shown to have the potential to significantly increase the high load limit compared to regular pump gasoline. Yang et. al. (SAE Int. J. Engines, Vol. 5, Issue 3, p. 1075, 2012) demonstrate that in HCCI operation a naphtha fuel (called “Hydrobate”) having a RON of 66 (and an AKI=67.5==(RON+MON)/2) provides a significantly higher load (and thus power) than gasoline having an AKI=87 and requires a much lower intake temperature. However, the low-octane gasolines are not readily available in the market.
Another approach for limiting the high pressure rise rates and subsequent knocking is to introduce some stratification so that the air and fuel are not perfectly mixed (i.e., not homogeneous). Stratification can be introduced in a number of different ways, including (1) injecting fuel and air separately and at different times so that only part of the fuel is premixed with air; (2) multiple injections of fuel some of which can mix with the air and some without and (3) injection of two fuels having different ignition characteristics. These advanced combustion approaches are different than HCCI and are known by several names including: Partially Premixed Combustion (PPC), Stratified Charge HCCI, and Dual Fuel Reactivity Controlled Compression Ignition (RCCI). Many of those approaches also seem to perform best with fuels having lower octane than pump gasoline.
For example, U.S. Patent Application Publication Number 2011/0271925 (“the '925 application”) discloses fuel compositions that yield very low soot and low NOx emissions while having high efficiencies and acceptable maximum in-cylinder pressure rise rates over a wide load range when used in an advanced combustion engine environment, especially one operating in partially-premixed combustion (PPC) mode. The fuel compositions disclosed in the '925 application have a boiling range of between 95 to 440 degrees Fahrenheit, and (a) a total sum of n-paraffins and naphthenes content of at least 7 volume percent and (b) a preferred RON of about 80 or less.
Another example is U.S. Patent Application Publication Number 20120012087 (“the '087 application”) which discloses fuel compositions that provide: (a) a significant reduction in NOx, (b) a reduction in soot emissions, and (c) high efficiencies, especially when compared to conventional diesel fuel compositions, when the fuels of that invention are employed in a partially premixed combustion mode in an advanced combustion engine. The fuel compositions disclosed in the '087 application have a boiling range of between 95 to 440 degrees Fahrenheit, and (a) a total sum of n-paraffins and naphthenes content of at least 22 volume percent and (b) a RON of about 90 or less. The best performing fuels had a RON of 80 or less.
A significant drawback of the use of fuels such as naphthas having a lower octane than pump gasoline is that they are present in refineries in much smaller quantities than gasoline and availability for sale at fuel stations would require additional fuel storage tanks which most fuel stations do not have space for. It would therefore be more advantageous, and cost effective, if the conventional pump gasoline could also be formulated to work in these engines.
One approach is to use additives to change the reactivity of pump gasoline. Cetane improvers such as 2-ethylhexyl nitrate (EHN) and di-tert butyl peroxide (DTBP) have typically been added to diesel fuels to increase their cetane number. However, the use of cetane improvers in conventional pump gasolines is limited, particularly in HCCI processes. For example, SAE Paper 2003-01-3170 by Eng et. al. discloses the use of DTBP to lower the low load stability limit in an HCCI single cylinder engine operated with PRF85 (a mixture of 85% iso-octane and 15% n-heptane, which by definition has a RON=MON=(RON+MON)/2=85. These types of PRF's are frequently used in research to represent gasoline. However, gasoline is known to be a more complex mixture and does not always perform the same as PRF. Further, they disclose that “adding an ignition promoter to extend the lower fueling rate limit” (i.e., the low load limit) “will result in a corresponding decrease in the maximum fueling level” (i.e., the high load limit).
Another example is SAE paper 2011-01-0361 by Hanson et. al which discloses the addition of EHN to gasoline to lower the low load limit in Reactivity-Controlled Compression Ignition (RCCI). RCCI utilizes two fuels with different reactivities and multiple fuel injections (one port and the other direct injection to create some stratification) to control air-fuel mixture reactivity in engine cylinders.
Combustion and Flame publication (132, (2003), 291-239) by Tanaka et. al. added 0.5 to 2% DTBP and EHN to PRF90 (90% iso-octane+10% n-heptane) and tested fundamental combustion behavior in a rapid compression machine (not an engine). Tanaka et. al. found that the cetane improvers shortened the ignition delay time (i.e., speed up the start of combustion). In addition, Tanaka et. al. reported that DTBP is more effective than EHN and that they do not see any change in burn rate which they relate directly to knocking behavior. Thus one would not expect an increase in the high load knocking limit.
Heretofore, there has been no appreciation or recognition that the addition of one or more cetane improvers to conventional pump gasoline can increase the high load limit in an internal combustion engine operated in a low temperature combustion mode thereby increasing the knock limit of the engine when operated in a low temperature combustion mode.